Abstract

The nonlinear evolution of magnetized Keplerian shear flows is simulated
in a local, three-dimensional model, including the effects of
compressibility and stratification. Supersonic flows are initially
generated by the Balbus-Hawley magnetic shear instability. The resulting
flows regenerate a turbulent magnetic field which, in turn, reinforces
the turbulence. Thus, the system acts like a dynamo that generates its
own turbulence. However, unlike usual dynamos, the magnetic energy
exceeds the kinetic energy of the turbulence by a factor of 3-10. By
assuming the field to be vertical on the outer (upper and lower)
surfaces we do not constrain the horizontal magnetic flux. Indeed, a
large-scale toroidal magnetic field is generated, mostly in the form of
toroidal flux tubes with lengths comparable to the toroidal extent of
the box. This large-scale field is mainly of even (i.e., quadrupolar)
parity with respect to the midplane and changes direction on a timescale
of ˜30 orbits, in a possibly cyclic manner. The effective
Shakura-Sunyaev alpha viscosity parameter is between 0.001 and 0.005,
and the contribution from the Maxwell stress is ˜3-7 times larger
than the contribution from the Reynolds stress.

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